The additional challenge, however, is that in the 36%-50% K-spar range, I believe the feldspar is still monoclinic. So while the triclinic 10%-36% K-spar range can be easily justified as albite, the Na-rich portion of sanidine may not quite be sanidine, but it isn't quite albite either... maybe it's really "K-stabilized monalbite"?  heh heh.

I've analyzed volcanic "sanidines" with Na>K right that plot right at the 36% K-spar boundary (for example, from https://www.mindat.org/loc-190531.html ); that I almost exactly landed on the mysterious boundary I found both intriguing and somehow mineralogically telling.

Under the microscope, I can't see any evidence of anorthoclase's characteristic twinning in the particular 36% K-spar content Azores feldspar example I looked at (it may be present sub-microscopically, although I didn't see any twinning under the microprobe either):

Without evidence of twinning, I'm assuming this material is still monoclinic, and so far, I've still called it "Na-dominant sanidine" based on the 36%-100% K-spar range. But if there's a better option for a name, I'm open to seeing how how this thread evolves.

Yes it can be hard to try to hammer mineral definitions based on optics over chemistry, into modern nomenclature.  I’m still not convinced we need orthoclase, but I think people are scared to move this motion.  I recently analysed some Sanidine/Anorthoclase crystals which were as low as 37mol% K, so was unsure what to call them, as sanidine really didnt suit the Na-rich chemistry. I eventually settled on sanidine/Anorthoclase, though maybe it should really have been high albite or analbite?